Who is Leggatt really?

When I first started reading The Secret Sharer by Joseph Conrad I believed Leggatt was a figment of the Captain's imagination. He was stuck at sea because there was no wind so it wasn't unusual for him to go a little crazy. The Captain and Leggatt would whisper to each other in bed and hang out in the Captain's room all day together and to me that was just crazy man talk, making somebody up to pass the time on a lonely ship where you were the stranger. That is until Ellie jokingly said in class that these two men were in a romantic relationship and Mr. Heidkamp agreed. After that whenever I picked up the book I could only picture these two men quickly falling in love with each other. It does make sense though, the men would whisper in bed together and the Captain was horrified of the fact that somebody would discover Leggatt. If he was just a stowaway I don't believe it would have been that big of a deal to the rest of the ship. At their devastating goodbye the men held onto each other in a long embrace never wanting to let go but knowing that if anybody found out about their relationship they would be in trouble because their relationship was not accepted at that time.